1. Field of the Invention
This invention relates to reflective liquid crystal light valve systems, and more particularly concerns improvements in the photosensitive layer of a liquid crystal light valve.
2. Description of Related Art
The liquid crystal light valve (LCLV) is a thin film multi-layer structure comprising a liquid crystal layer, a dielectric mirror, a light blocking layer, and a photosensitive layer, all sandwiched between two transparent electrodes. In a typical reflective LCLV projection system a high intensity polarized projection beam is directed through the liquid crystal layer to the dielectric mirror. An input image of low intensity light, such as that generated by a cathode ray tube, is applied to the photosensitive layer, thereby switching the electric field across the electrodes from the photosensitive layer onto the liquid crystal layer. This field selectively activates the liquid crystal according to the intensity of input light received at different small areas or pixels of the photosensitive layer. Linearly polarized projection light from a high power light source, such as a xenon arc lamp, passes through the liquid crystal layer and is reflected from the dielectric mirror. Light reflected from the mirror is polarization modulated by the liquid crystal in accordance with the light information incident on the photosensitive layer. Therefore, if a complex distribution of light, for example a high resolution input image from a cathode ray tube, is focused onto the photosensitive layer, the liquid crystal light valve converts the low intensity input image into a replica image which can be reflected for projection with magnification to produce a high brightness image on a larger viewing screen. Projection systems of this type are described in several U.S. Pat. Nos., including U.S. Pat. No. 4,650,286 to Koda, et al for Liquid Crystal Light Valve Color Projector; 4,343,535 to Bleha, Jr. for Liquid Crystal Light Valve; 4,127,322 to Jacobson, et al for High Brightness Full Color Image Light Valve Projection system; and 4,191,456 to Hong, et al for Optical Block For High Brightness Full Color Video Projection System.
Resolution, image clarity and other parameters of the projected image of the liquid crystal light valve projection system depend to a high degree on the nature and operation of the photosensitive layer. The photosensitive layer may be made of cadmium sulfide, amorphous silicon, or single crystal silicon. In the case of single-crystal silicon light valves the prior art includes a regular array of very small diodes which effectively isolate the individual pixels of the input image from one another. In one form of photosensitive layer a plurality of diodes in a regular pattern are formed on a silicon substrate in potential wells associated with each of the closely spaced diodes. Each diode corresponds to an individual pixel or image area and, in some of the prior art devices, each may be a square of about 18 microns on each side, with each of the squares of the regular diode pattern being separated from adjacent squares by a gap of 2 to 3 microns. As the photosensitive layer experiences the AC bias that is applied to the electrodes of the LCLV, the silicon layer below each diode is alternately driven into depletion and into accumulation. In depletion mode, photogenerated charges caused by the input image are swept through the body of the material to the diode array. The purpose of the diode array is to localize passage of the charge at the surface of the photosensitive layer, and to limit motion of such charge laterally along the surface, thereby maintaining the resolution provided by the diode pattern. The resulting spatially varying impedance pattern at the photosensitive layer produces a corresponding increase in voltage dropped across the liquid crystal layer in a spatially varying pattern that matches the input image. This produces a corresponding two dimensional birefringence pattern in the liquid crystal material, having a magnitude that follows the input image. Birefringence is read out with a standard polarizing biprism to allow projection of the output image by means of the high intensity reading light.
Resolution of such a photosensitive layer is dependent upon the size of the individual diodes. Therefore, resolution no better than that allowed by the smallest diode size can be achieved.
Further, the regularity of the pattern of diodes tends to cause certain input image types to produce artificial artifacts in the displayed image. These artifacts are caused by interactions between the structure inside the LCLV and the input information. For example, if the input image should contain elements having a spatial repetition rate that is the same as or close to the spatial repetition rate of the diodes of the photosensitive layer, a pattern of alternate light and dark areas or a type of banding effect may be produced in the projected image. As a rough example of this effect, consider a series of parallel closely spaced slits in a mask which receives a pattern of light having similar parallel alternate strips of light and dark but in which the spacing of the light bands is somewhat different than that of the mask slits. At certain points the light areas or strips of the incoming information are aligned with the slits in the mask, whereas at other points the light areas or strips of the input image are blocked by the space between mask slits. This effect repeats at relatively low spatial frequency, resulting in a banding artifact in the image of light that passes through the slit mask. Similarly, if the input image of the liquid crystal light valve should have components that have a spatial pattern or spatial frequency analogous to the regular spatial pattern and spatial frequency of the diode array, artificial artifacts will be provided. In other words, the regular pattern of diodes in the photosensitive layer may produce information in the output display that is not in the original image but is solely the result of interaction between information contained in the image and information superimposed by the pattern of the diode array.
An additional problem with present arrangements for manufacture of photosensitive layers for the LCLV involves the requirement for use of complex masking steps for forming the diode array. These steps demand increased handling operations, more capital equipment and potential contamination of substrates by materials used in the photolithography process.
Accordingly, it is an object of the present invention to provide a liquid crystal light valve that avoids or minimizes above mentioned problems.